Wells are generally drilled into the ground to recover natural deposits of hydrocarbons and other desirable materials trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a “drill string.” The drill string is a long string of sections of drill pipe that are connected together end-to-end. Drilling fluid, or mud, is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in the annulus between the drill string and the borehole wall.
In conventional drilling, a well is drilled to a selected depth, and then the wellbore is typically lined with a larger-diameter pipe, usually called casing. Casing typically consists of casing sections connected end-to-end, similar to the way drill pipe is connected. To accomplish this, the drill string and the drill bit are removed from the borehole in a process called “tripping.” Once the drill string and bit are removed, the casing is lowered into the well and cemented in place. The casing protects the well from collapse and isolates the subterranean formations from each other.
Conventional drilling typically includes a series of drilling, tripping, casing and cementing, and then drilling again to deepen the borehole. This process is very time consuming and costly. Additionally, other problems are often encountered when tripping the drill string. For example, the drill string may get caught up in the borehole while it is being removed. These problems require additional time and expense to correct.
FIG. 1A shows a prior art drilling operation. A drilling rig 101 and rotary table 104 at the surface are used to rotate a drill string 103 with a drill bit 105 disposed at the lower end of the drill string 103. The drill bit 105 drills a borehole 107 through subterranean formations that may contain oil and gas deposits. Typically, an MWD (measurement while drilling) or LWD (logging while drilling) collar 109 is positioned just above the drill bit 105 to take measurements relating to the properties of the formation as the borehole 107 is being drilled. In this description, MWD is used to refer either an MWD system or an LWD system. Those having ordinary skill in the art will realize that there are differences between these two types of systems, but the differences are not germane to the embodiments of the invention.
The term “casing drilling” refers to using a casing string as a drill string when drilling. A bottom hole assembly (“BHA”), including a drill bit, is connected to the lower end of a casing string, and the well is drilled using the casing string to transmit drilling fluid, as well as axial and rotational forces, to the drill bit. Casing drilling enables the well to be simultaneously drilled and cased.
FIG. 1B shows a prior art casing drilling operation. A rotary table 124 at the surface is used to rotate a casing string 123 that is being used as a drill string. The casing 123 extends downwardly into borehole 127. A drill bit 125 is connected to the lower end of the casing string 123. When drilling with casing, the drill bit 125 must be able to pass though the casing string 123 so that the drill bit 125 may be retrieved when drilling has been completed or when replacement or maintenance of the drill bit 125 is required. Thus, the drill bit 125 is sized smaller than the inner diameter of the casing string 123.
The drill bit 125 drills a pilot hole 128 that must be enlarged so that the casing string 123 will be able to pass through the borehole 127. An underreamer 124 is positioned below the casing string 123 and above the drill bit 125 so as to enlarge the pilot hole 128. A typical underreamer 124 can be positioned in an extended and a retracted position. In the extended position, the underreamer 124 enlarges the pilot hole 128 to the underreamed borehole 127, and in the retracted position (not shown), the underreamer 124 collapses so that it is able to pass through the inside of the casing string 123.
FIG. 1B also shows an MWD collar 135 positioned above the drill bit 125 and the underreamer 124, but below the casing string 123. The MWD collar 135 takes measurements related to formation properties as drilling is taking place.
Casing drilling eliminates the need to trip the drill string before the well is cased. The drill bit may simply be retrieved by pulling it up through the casing. The casing may then be cemented in place, and then drilling may continue. This reduces the time required to retrieve the BHA and eliminates the need to subsequently run casing into the well.
Another aspect of drilling is called “directional drilling.” Directional drilling is the intentional deviation of the wellbore from the path it would naturally take. In other words, directional drilling is the steering of the drill string so that it travels in a desired direction.
Directional drilling is advantageous in offshore drilling because it enables many wells to be drilled from a single platform. Directional drilling also enables horizontal drilling through a reservoir. Horizontal drilling enables a longer length of the wellbore to traverse the reservoir, which increases the production rate from the well.
A directional drilling system may also be used in vertical drilling operation as well. Often the drill bit will veer off of an planned drilling trajectory because of the unpredictable nature of the formations being penetrated or the varying forces that the drill bit experiences. When such a deviation occurs, a directional drilling system may be used to put the drill bit back on course.
One method of directional drilling uses a bottom hole assembly (“BHA”) that includes a bent housing and a mud motor. A bent housing 200 is shown in FIG. 2A. The bent housing 200 includes an upper section 203 and a lower section 204 that are formed on the same section of drill pipe, but are separated by a bend 201. The bend 201 is a permanent bend in the pipe.
With a bent housing 200, the drill string is not rotated from the surface.
Instead, the drill bit 205 is pointed in the desired drilling direction, and the drill bit 205 is rotated by a mud motor (not shown) located in the BHA. A mud motor converts some of the energy of the mud flowing down through the drill pipe into a rotational motion that drives the drill bit 205. Thus, by maintaining the bent housing 200 at the same azimuthal position with respect to the borehole, the drill bit 205 will drill in the desired direction.
When straight drilling is desired, the entire drill string, including the bent housing 200, is rotated from the surface. The drill bit 205 angulates with the bent housing 200 and drills a slightly overbore, but straight, borehole (not shown).
Another method of directional drilling includes the use of a rotary steerable system (“RSS”). In an RSS, the drill string is rotated from the surface, and downhole devices cause the drill bit to drill in the desired direction. Rotating the drill string greatly reduces the occurrences of the drill string getting hung up or stuck during drilling.
Generally, there are two types of RSS's—“point-the-bit” systems and “push-the-bit” systems. In a point-the-bit system, the drill bit is pointed in the desired direction of the borehole deviation, similar to a bent housing. In a push-the-bit system, devices on the BHA push the drill bit laterally in the direction of the desired borehole deviation by pressing on the borehole wall.
A point-the-bit system works in a similar manner to a bent housing because a point-the-bit system typically includes a mechanism for providing a drill bit alignment that is different from the drill string axis. The primary differences are that a bent housing has a permanent bend at a fixed angle, and a point-the-bit RSS has an adjustable bend angle that is controlled independent of the rotation from the surface.
FIG. 2B shows a point-the-bit RSS 210. A point-the-bit RSS 210 typically has an drill collar 213 and a drill bit shaft 214. The drill collar 213 includes an internal orientating and control mechanism (not shown) that counter-rotates relative to the drill string. This internal mechanism controls the angular orientation of the drill bit shaft 214 relative to the borehole (not shown).
The angle θ between the drill bit shaft 214 and the drill collar 213 may be selectively controlled. The angle θ shown in FIG. 2B is exaggerated for purposes of illustration. A typical angle is less than 2 degrees.
The “counter rotating” mechanism rotates in the opposite direction of the drill string rotation. Typically, the counter rotation occurs at the same speed as the drill string rotation so that the counter rotating section maintains the same angular position relative to the inside of the borehole. Because the counter rotating section does not rotate with respect to the borehole, it is often called “geo-stationary” by those skilled in the art. In this disclosure, no distinction is made between the terms “counter rotating” and “geo-stationary.”
A push-the-bit system typically uses either an internal or an external counter-rotation stabilizer. The counter-rotation stabilizer remains at a fixed angle (or geo-stationary) with respect to the borehole wall. When the borehole is to be deviated, an actuator presses a pad against the borehole wall in the opposite direction from the desired deviation. The result is that the drill bit is pushed in the desired direction.
FIG. 2C shows a typical push-the-bit system 220. The drill string 223 includes a counter-rotating collar 221 that includes one or more extendable and retractable pads 226. Because the pads 226 are disposed on the counter-rotating collar 221, they do not rotate with respect to the borehole (not shown). When a pad 226 is extended into contact with the borehole (not shown) during drilling, the drill bit 225 is pushed in the opposite direction, enabling the drilling of a deviated borehole.
FIG. 3 shows a prior art drilling system that includes both casing drilling and directional drilling. A rotary table 304 is used to rotate a casing string 311 that is being used as a drill string. A drill bit 305 and an underreamer 313 are positioned at the lower end of the casing string 311. The drill bit 305 drills a pilot hole 308 that is enlarged to an underreamed borehole 307 by the underreamer 313.
The casing drilling system also includes an RSS 315 that is positioned blow the casing string 311 and between the drill bit 305 and the underreamer 313. The RSS 315 is used to change the direction of the drill bit 305.
Nonetheless, a need still exists for an improved drilling system.